Laser melting of filler metal is used for additive manufacturing and repair of articles including gas turbine components. Flux can be introduced via a flux core in a wire of filler metal or a flux coating on a filler wire or as granulated flux material fed in conjunction with solid filler metal in wire or strip form. Alternately, the filler metal and flux can be provided in powder form. Powder is particularly advantageous because laser energy is more readily captured by powder than by solid filler metal. A disadvantage of wire feed is that the laser beam must continually focus on the wire while simultaneously feeding and moving the wire in precise alignment with the laser. Wire cannot be moved as quickly as a laser beam scanned by pivoting mirrors. Thus, wire limits the processing speed.
Filler powder delivery options include:
(1) Powder sprayed to the point of processing. This results in scattering and waste of powder. Even on flat horizontal surfaces, the net capture of sprayed powder is only about 65%. Powder spray nozzles cannot move as quickly as a laser beam, so nozzles limit the processing speed.
(2) Preplacement of powdered metal and powdered flux in separate layers. This controls the filler thickness distribution and the metal/flux ratio. However, it is labor intensive and time consuming
(3) Preplacement of a mixture of powdered metal and powdered flux. This reduces labor by about half over placing metal and flux in separate layers. However, it is difficult to ensure uniform mixing due to differences in size and density of the respective powders. Use of composite metal/flux powder avoids such mixing and segregation issues, but composite preparation is expensive and time consuming
(4) Capturing metal and flux powders in a sacrificial sleeve or packet that is placed in the path of processing is a good option because powder waste is minimized and precise distribution of powder is possible. This method is relatively quick and requires little labor. However, mixed metal and flux powders in a sleeve can still segregate, resulting in inconsistent metal/flux ratios across the powder volume and the working surface area.